The conventional electrochemical devices using liquid electrolytes had safety problems such as leakage and explosion and this led into the development of electrochemical devices using solid electrolytes. Lithium-polymer batteries, as one of such electrochemical devices using solid electrolytes, has been shown very advantageous in that they provide improved safety, are very economical due to highly efficient recharge/discharge rates, provide diverse designs and can provide miniaturization of batteries by manufacturing them in thin films. In particular, it has been drawing much attention to use a polyalkyleneoxide(PAO)-based solid polymer as an electrolyte because it can lead to the manufacture of batteries with much improved energy density. PAO-based solid polymer was first suggested in 1975 by P. V. Wright (British Polymer Journal Vol. 7, p. 319) and named as an ion-conductive polymer by M. Armand in 1978, and its applications has been expanded thereafter.
In general, a PAO-based solid polymer electrolyte consists of a complex between a polymer which contains electron-donating atoms such as O, N, and P and a lithium salt, for example, a complex between polyethylene oxide (PEO) and a lithium salt. This can be used as an electrolyte in electrochemical devices for operation at high temperature. However, its ionic conductivity at room temperature is as low as 10−8 S/cm and thus it has not been able to apply for operations at room temperature. It is essential to increase the amorphous region of electrolytes to improve its ionic conductivity at room temperature and numerous studies have been attempted to improve the ionic conductivity by reducing the crystallinity of polymer electrolytes.
Examples of the above-mentioned methods are: a method of blending an electrolyte with an amorphous polymer, decreasing crystallinity of a polymer backbone by adding a plasticizer (Electrochim. Acta, 40, 2315 (1995)), a method of enhancing ionic conductivity of an electrolyte by reducing crystallinity by binding a low molecular weight ethylene oxide as a side chain to the main backbone of the atypical polymer (J. Electrochem. Soc., 144, 1174 (1997), J. Polym. Sci., Part A, Polym. Lett., 28, 187 (1990)), a method of enhancing ionic conductivity of an electrolyte by grafting a low molecular weight PEO to a polymer having a network structure (Eur. Polym. J., 29, 799 (1993)), a method of manufacturing cross-linked polymer electrolytes from a composition, which comprises acrylate of polyalkyleneglycol having an unsaturated functional group and is mixed with an ion conductive liquid or electrolyte salt, by curing via UV or electron-beam irradiation (U.S. Pat. No. 4,830,939, J. Electrochem. Soc., 145, 1521 (1998)) and the like. However, these solid polymer electrolytes with comb-type or network structures have ionic conductivity at room temperature as low as about 10−5 to 10−4 S/cm and also had poor mechanical properties when formed in films.
The studies for improving ionic conductivity and mechanical properties of electrolytes have been pursued, and as a result, a solid polymer electrolyte with an improved ionic conductivity was developed by adding a low molecular weight polyethylene oxide to a vinylfluoride-hexafluoropropane copolymer (Chem. Mater., 9 (1997) 1978). Further, it was reported that an electrolyte was improved of its inflamability and ionic conductivity at room temperature to 4×10−5 S/cm using siloxane polymer, wherein polyethylene oxide is grafted as a side chain, as a plasticizer thereby inducing to have a structure of an interpenetrating polymer network (IPN) with polyethylene glycol diacrylate (Electrochim. Acta, 48, 2215 (2003), J. Power Sources, 119-121, 442 (2003)). Besides, the inventors of the present invention disclosed solid polymer electrolytes with improved mechanical properties by using a cross-linker wherein three ethyleneglycol acrylates are introduced to the center of cyclic alkyl, heterocyclic molecules (Korean Pat. Nos. 298,802 & 394,077).
Polysiloxane-based polymers have relatively high plasticity and low glass transition temperature and are thus expected to improve the segmental motion of polymer chain. Therefore, there have been studies recently on how to apply those polysiloxane-based polymers to the basic backbone of the crosslinkable and comb-type polymer electrolytes of polyalkyleneoxide (Macromol. Chem. Rapid Commun., 7 (1986) 115, Macromolecules, 36 (2003), 9176, U.S. Pat. Nos. 4,673,718, 4,766,185, 5,227,043, 5,440,011, Japanese Laid-Open Hei 5-290616).
However, the polysiloxane-based polymers are known disadvantageous in that they have poor mechanical properties, low ionic conductivity of 10−4 S/cm thus not suitable to be used for lithium batteries which are used at room temperature.
As a way to solve the above problems, the inventors of the present invention have previously disclosed a method to remedy the poor mechanical properties of the polysiloxane-based polymers by using a novel cross-linker wherein a crosslinkable acryl group is introduced to both terminal ends of methylsiloxane polymer, having polyalkyleneoxide introduced as a side chain, by means of heat or light (Korean Pat. No. 419,864). Although the polymer electrolyte composition disclosed in the above Korean patent was remarkable it had also shortcomings that the cross-linking density and the mechanical properties of thus obtained electrolytes were not easy to control upon necessity because the acryl group is present only at terminal ends of methylsiloxane polymer.